When interconnects adjacent to one another are formed, it is considered to form air gaps between the interconnects to reduce the capacitance between the interconnects. An example of such interconnects is bit lines in a NAND memory. When the air gaps are formed between the interconnects, it is desirable to use such a method of forming the air gaps that can further largely reduce the capacitance between the interconnects. The same holds true for the case where the air gaps are formed between conductors other than the interconnects.